Operando and in situ insights into cobalt selenide cathodes for rechargeable aluminum batteries: Conversion mechanism, degradation pathways and electrolyte interactions

Cobalt selenide (CoSe) has been explored as a cathode material for rechargeable aluminum batteries, particularly under high current densities (1 A g-1) with a reported capacity of near 250 mAh g-1 at 5 A g-1. However, its rapid capacity fading has hindered practical applications, and the underlying mechanisms proposed to be attributed to the intercalation of Al3+ remain unclear. In this study, CoSe was synthesized via high-temperature selenization of a ZIF-67 MOF template, and its electrochemical behavior was investigated using operando, in situ and ex situ characterization techniques. Contrary to the previously accepted Al3+ intercalation mechanism, we reveal a complex, partially reversible conversion mechanism driven by selenium redox activity. Operando synchrotron diffraction and X-ray absorption spectroscopy show a phase transition from hexagonal CoSe to cubic CoSe2, alongside selenium oxidation from Se2- to (Se2)2-. Cobalt remains electrochemically inactive but undergoes structural reorganization and dissolution, forming soluble chloroaluminate complexes that migrate to the Al anode. This process contributes to capacity fade and anode contamination, as confirmed by ex situ SEMEDX and ICP-OES. At high current densities, the desired electrochemical reactions outpace parasitic side reactions, preserving activity. These findings redefine the charge storage mechanism in CoSe and provide a framework for designing more stable cathodes for long-term RAB performance.